if_muge: Add GMII enable (vs RGMII) bit

The GMII control bit ETH_MAC_CR_GMII_EN_ is not documented in
LAN78xx datasheets, but from the permissively licensed header provided
by Microchip it is:

#define ETH_MAC_CR_GMII_EN (0x00080000UL ) // GMII/RGMII Selection